In aqueous ink indirect printing, an aqueous ink is jetted onto an intermediate imaging surface, which can be in the form of a blanket. The ink may be dried or partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality, it is desirable that the ink drops jetted onto the blanket spread and become well-coalesced prior to drying. Otherwise, the ink images may appear grainy and/or have deletions. Lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink may be facilitated by materials having a high surface free energy, and therefore it is desirable to use a blanket having a high surface free energy to enhance ink spreading.
However, in order to facilitate transfer of the ink image from the blanket to the media substrate after the ink is dried or partially dried on the intermediate imaging surface, a blanket having a surface with a relatively low surface free energy is preferred. Rather than providing the desired spreading of ink, low surface energy materials tend to promote “beading” of individual ink drops on the image receiving surface.
Thus, an optimum blanket for an indirect image transfer process should tackle all of the challenges of wet image quality, including desired spreading and coalescing of the wet ink, and the image transfer of the dried or partially dried ink. The first challenge—wet image quality—prefers a high surface energy blanket that causes the aqueous ink to spread and wet the surface. The second challenge—image transfer—prefers a low surface energy blanket so that the ink, once dried, has minimal attraction to the blanket surface and can be transferred to the media substrate. Those two conflicting requirements can make the whole process of wetting, release, and transfer in indirect printing processes very challenging.
In addition to indirect ink jet printing, offset lithography is a common method of printing today and, having similar challenges, is contemplated for the processes and compositions disclosed herein. In a typical lithographic process, a printing plate, which may be a flat plate, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a fountain solution (for example comprising water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel fountain solution and accept ink, whereas the fountain solution formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder may be covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder helps to deliver a more uniform layer of printing material to the substrate free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset cylinder to the substrate. Pinching the substrate between the offset cylinder and an impression cylinder may provide this pressure.
In one variation, referred to as dry or waterless lithography or driography, the plate cylinder is coated with a silicone rubber that is hydrophobic and physically patterned to form the negative of the printed image. A printing material is applied directly to the plate cylinder, without first applying any fountain solution as in the case of the conventional or “wet” lithography process described earlier. The printing material includes ink that may or may not have some volatile solvent additives. The ink is preferentially deposited on the imaging regions to form a latent image. If solvent additives are used in the ink formulation, they may preferentially diffuse towards the surface of the silicone rubber, thus forming a release layer that may reject the printing material. The low surface energy of the silicone rubber adds to the rejection of the printing material. The latent image may again be transferred to a substrate, or to an offset cylinder and thereafter to a substrate, as described above.
The above-described inkjet and lithographic printing techniques may have certain disadvantages. For example, one disadvantage encountered in attempting to modify conventional lithographic systems for variable printing is a relatively low transfer efficiency of the inks off of the imaging plate or belt. For example, in some instances, about half of the ink that is applied to the “reimageable” surface actually transfers to the image receiving media substrate requiring that the other half of the ink be cleaned off the surface of the plate or belt and removed. This relatively low efficiency compounds the cleaning problem in that a significant amount of cleaning may be required to completely wipe the surface of the plate or belt clean of ink so as to avoid ghosting of one image onto another in variable data printing using a modification of conventional lithographic techniques.
Also, unless the ink can be recycled without contamination, the effective cost of the ink is doubled. Traditionally, however, it is very difficult to recycle the highly viscous ink, thereby increasing the effective cost of printing and adding costs associated with ink disposal. Proposed systems fall short in providing sufficiently high transfer ratios to reduce ink waste and the associated costs. A balance must therefore be struck in the composition of the ink to provide optimum spreading on a plate or belt surface including adequate separation between printing and non-printing areas and an increased ability to transfer to a substrate.
Various approaches have been investigated to provide potential solutions to balance the above-mentioned challenges. Those approaches include, for example, blanket material selection, ink design, and auxiliary fluid methods. With respect to blanket material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, a fluoropolymer, such as Teflon®, Viton®, and certain hybrid materials. Those materials may have a relatively low surface energy, but may provide poor wetting. Alternatively, polyurethane and polyimide have been used to improve wetting, but at the cost of ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it may not jet properly. If, however, the ink surface tension is too low, it will drool out of the face plate of the print head.
One solution that has been proposed is applying a sacrificial wetting enhancement coating, such as a sacrificial coating composition comprising polyvinyl alcohol or starch, onto the blanket. The sacrificial coating composition may be applied to the intermediate transfer member (blanket), where it dries to form a solid film. The coating can have a higher surface energy and/or be more hydrophilic than the base intermediate transfer member. Droplets of ink may be ejected in an imagewise pattern onto the sacrificial coating composition, and then the ink may be at least partially dried to form an ink pattern on the blanket. Finally, the ink pattern and the sacrificial coating composition may be transferred from the blanket to a substrate, such as paper.
Both polyvinyl alcohol and starches, however, are known adhesives. Accordingly, sacrificial coating compositions comprising polyvinyl alcohols and/or starches may have a high release force when coated onto a blanket. This high release force may result in paper jams and/or stripping of the ink during the printing process, as the polyvinyl alcohol or starch based sacrificial coating composition adheres to the blanket.
In order to implement a polyvinyl alcohol or starch based sacrificial coating composition that does not undesirably adhere to the blanket, it is desirable to lower the high release force observed in such sacrificial coating compositions while still maintaining their beneficial properties, such as good wet image quality, for use in indirect printing processes.